How feasible is it to use aluminum and liquid oxygen as future propellant sourced from the moon?

Question

I was looking into whether there would be any way to source propellants from the moon itself in support of a future base and space stations in cis-lunar space. I found two documents exploring how aluminum in particular might work in combination with liquid oxygen.

One is by Wickman Spacecraft & Propulsion, who say:

An additional option available with aluminum is to suspend the aluminum powder in gelled LOX to form a monopropellant... As part of our research, we made a small rocket engine fueled by the LOX-aluminum monopropellant... The propellant tank was surrounded by a liquid nitrogen bath to keep the LOX from boiling off. The propellant feed lines ran through a liquid nitrogen bath on their way to the combustion chamber. A piston pushed against the propellant to feed the propellant into the rocket engine chamber. While the thrust was only about a pound, the engine was started and stopped several times without a flashback of the combustion flame front into the propellant tank.

The other is from a document called the Moon Miner's Manifesto, hosted on the website of the Artemis Society International:

Aluminum and oxygen alone will provide a specific impulse somewhat lower than most hydrocarbons. Brower et al. expect a value of 285 seconds... One [engine design] would be to pump aluminum powder as we do fluids. In this case, it will probably be necessary to use a carrier gas along with the powder to keep the aluminum grains from vacuum welding or sticking together from electrostatic forces... Another technique is a hybrid rocket engine using solid aluminum and liquid oxygen. A conceptual design for such an engine was proposed by Brower et al. Their design calls for a hexagonal array of aluminum bars the length of the combustion chamber. Liquid oxygen would be fed down the bars for regenerative cooling before reaching the flame at the bar tips. The engine could use oxygen and aluminum only, or could use tripropellant operation with hydrogen.

The Brower et al. paper was presented at the 26th Joint Propulsion Conference in 1990 and is in the catalogue of the AIAA.

Obviously this needs a great deal of development, but at first blush it seems like it could have advantages over the usual proposal at this point - acquiring water ice from asteroids, or from permanently shaded craters at the moon's poles (if present in suitable quantities and concentrations), and splitting it into hydrogen and oxygen for use in H₂/LOX engines.

The lunar highlands are composed largely of anorthite (CaAl₂Si₂O₈), a mineral that is 15% aluminum and 62% oxygen. Regions that are 80% anorthite or more seem to be common. Extraction of oxygen has been discussed in a number of papers, it requires heating regolith above 1100 Celsius so that the mineral oxides dissociate and the oxygen can be collected.

Edit: I asked about this on the SE Chemistry community and got an answer from Jon Custer:

Fine powder feedstock... into an arc discharge to vaporize and ionize... electrostatic acceleration, then electromagnets to mass separate. Power with a solar panel array. No moving parts (except for the powder feed). No vacuum systems needed. And you separate all of the elements at once.

It has been pointed out to me by TildalWave that this would require a great deal of electricity and coolant. It occurred to me that perhaps the coolant could be avoided by putting the whole separator mechanism on rails, on a long shaded track running beside your solar panel field. If you set up the separator to dump the metal and slag products directly onto the ground under the track as it moves along, perhaps it is just a matter of figuring out how to get the flow and the rate at which the separator moves right, and the cooling could happen passively. There would be linear trails of metals and slag left on the ground which could slowly radiate away their heat until they are cool enough to be collected. There are probably reasons that wouldn't work, but I need to be told what they are. The oxygen that off-gasses could be collected from within the open-bottom chamber where the vaporized streams are ejected by the separator.

Could some process along these lines be competitive with asteroid mining? What are the issues and hurdles?

Answer 1

There are several issues at play here.

1. Are the raw materials needed to be found on the moon?
2. Can the raw materials be realistically harvested on the moon?
3. Can the raw materials be processed into a rocket propellant on the moon?
4. Are rocket engines which use this propellant feasible?
5. Are there more suitable materials for this purpose to be found on the moon?

Let's look at them in turn:

Are the raw materials needed to be found on the moon?

As per the OP's cited paper the materials exist in sufficient quantity on the moon. This is well confirmed via many sources, as is shown at Wikipedia.

Can the raw materials be realistically harvested on the moon?

Yes they can, and in fact were "harvested" by Apollo. Much refining would be required, but the raw materials can easily be harvested.

Can the raw materials be processed into a rocket propellant on the moon?

As per the paper cited above, the rocket propellant is a suspension of aluminum in LOX. Realistically, I do not foresee this suspension being used in actual production however the researchers do mention that this is not the only possible concoction. Perhaps in the moon's low-gravity, low-pressure, low-temperature environment an easier to form and easier to handle concoction may be feasible. Additional research is needed in this particular area.

Are rocket engines which use this propellant feasible?

Again using a paper cited by the OP the answer seems to be true. At the current state of technology (admittedly very early) it seems that only small, low power engines are possible. However, the moon is a low-gravity world with no appreciable atmosphere. Such a low-power application may be be enough for moon-station to moon-station hopping.

Are there more suitable materials for this purpose to be found on the moon?

Almost certainly. The electrolysis of water (used to acquire the oxygen) will produce hydrogen as a byproduct, as a trivial example. Water is limited in it's locations on the Moon, however, but can be found in some locations.

Conclusion

Based on the above, I conclude that given an established mining and manufacturing presence on the moon, and giving enough power, an aluminum-oxygen rocket engine might use locally-sourced fuel for jaunting around the moon. However, I do not expect that this technology could be used to power ships from the moon to other destinations.

Answer 2

I'm going to go out on a limb and suggest that the technology to refine the ore works, as well as the ability to make the rocket. Here's a few things for thought:

1. Assuming an ISP of, say, 200, a proper mixture of 1/3rd mass, 2/3rd fuel would be sufficient to achieve lunar orbit. For a 4/5 fuel ratio, you could return directly to Earth.
2. Producing the LOX at the Moon would be a fairly difficult problem, as it would need to be kept cool the entire time.
3. Solids have never really been used in rockets before (Aside from solid rockets, which is a different mechanism) This would be a hybrid type rocket, pumping solids, which seems challenging. Also, you'd have to have the aluminum as a dust, which might be abrasive.
4. This seems like a fairly risky thing to do, based primarily on #3, for a manned mission. You gain a relatively small weight boom, but with a much more risky rocket.

I could see this as a return stage for lunar mining mission, but not for a manned mission. ISRU just isn't as important for a Lunar mission as it would be for a Mars mission.